1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, which has a function for forming an image on a recording material such as a sheet.
2. Description of the Related Art
An image forming apparatus having such a structure as to use an intermediate transfer member is conventionally known as an image forming apparatus such as a copying machine and a laser beam printer. The image forming apparatus, firstly, transfers a toner image which has been formed on the surface of a photosensitive drum (image bearing member) onto an intermediate transfer member, in a primary transferring step. After that, the image forming apparatus repeats the primary transferring step on respective toner images of a plurality of colors, and thereby forms a toner image having the plurality of the colors on the surface of an intermediate transfer member. Subsequently, in a secondary transferring step, the image forming apparatus transfers the toner image having the plurality of the colors, which has been formed on the surface of the intermediate transfer member, collectively onto the surface of the recording material such as paper. On the recording material onto which the toner image has been collectively transferred, the toner image is permanently fixed by a fixing device, and thereby a full color image is formed. A residual toner (secondary transfer residual toner) which has remained on the intermediate transfer member after the secondary transferring step needs to be cleaned for the purpose of preparing for the next image formation. Japanese Patent Application Laid-Open No. H09-50167 proposes a cleaning method as follows. The cleaning method includes firstly applying an alternating voltage to a charging device using an electroconductive roller, and charging the secondary transfer residual toner existing on the intermediate transfer member after the secondary transferring step to a reverse polarity of the polarity of the toner, which has been charged in development. After that, the secondary transfer residual toner which has been charged to the reverse polarity moves to the photosensitive drum, and is collected by a cleaning unit on the photosensitive drum.
Furthermore, Japanese Patent Application Laid-Open No. 2009-205012 proposes a method of using an electroconductive brush (brush member) as a charging device, which comes in contact with the intermediate transfer member without being moved or rotated while the intermediate transfer member moves. Specifically, the electroconductive brush can approximately uniformly scatter and charge the secondary transfer residual toner on the intermediate transfer member. In contrast to the structure in Japanese Patent Application Laid-Open No. H09-50167, the structure in Japanese Patent Application Laid-Open No. 2009-205012 can uniformly charge the secondary transfer residual toner existing on the intermediate transfer member only with a direct voltage, without using the alternating voltage.
However, in the case where the electroconductive brush is adopted as the charging device, there is a concern that the charging function of the electroconductive brush deteriorates and consequently an image failure occurs due to the cleaning failure, when the number of printed sheets increases. When the electroconductive brush charges the secondary transfer residual toner, a part of the secondary transfer residual toner deposits on the electroconductive brush. When the number of the printed sheets has increased, the amount of the secondary transfer residual toners deposited on the electroconductive brush may increase, and the secondary transfer residual toner may accumulate in the electroconductive brush. When the amount of the secondary transfer residual toners deposited on the electroconductive brush increases, an apparent resistance of the electroconductive brush increases. In this case, a charging capability of the electroconductive brush decreases, and the electroconductive brush cannot sufficiently charge the secondary transfer residual toner existing on the intermediate transfer member. The secondary transfer residual toner which has been insufficiently charged receives a small force from an electric field in the periphery, and resists moving to the photosensitive drum when a potential difference is small between the surface of the photosensitive drum and the intermediate transfer member. Particularly, when the secondary transfer residual toner is moved to an image forming portion on the photosensitive drum, there is a possibility that the secondary transfer residual toner which has been insufficiently charged cannot move to the photosensitive drum and remains on the intermediate transfer member, because the photosensitive drum has been exposed and has small negative polarity. When the secondary transfer residual toner which has been insufficiently charged has remained on the intermediate transfer member together with an image which has been transferred from the photosensitive drum, there is a concern that an image failure occurs which originates in a phenomenon that a toner of the next image results in overlapping with the remaining secondary transfer residual toner of the prior image on the intermediate transfer member, which has not been completely collected.